The problem of air pollution is not a new one. However, the problem has become more and more serious in many cities in recent years. Most of the pollution is a result of compounds which are derived from unburned or partially burned hydrocarbons found in the exhaust of internal combustion engines.
It is well known that noble metal such as platinum and palladium and combination of these metals have been and are currently being used in catalysts for the control of auto exhaust emissions. A wide variety of of activities can be achieved dependent on the choice noble metal salt used in the preparation. Since the use of noble metal auto exhaust catalysts is controlled to a great extend by cost, small amounts of noble metals must be used to maximum advantage. This means that the location and distribution of noble metals are of utmost importance in determining the intrinsic activity of the catalyst.
Several patents have been issued that disclose and claim conversion of exhaust gases to innocuous entities in the presence of platinum and palladium catalysts. In addition to being the principal components of these catalysts several of the noble metals are used in small amounts to promote activity of base metal systems. U.S. Pat. No. 3,189,563 of Hauel, issused June 15, 1965, is typical of the patents relating to the use of noble metal catalysts for the conversion of automobile exhaust gases. U.S. Pat. No. 3,455,843 to Briggs et al, issued July 15, 1969, is typical of a base metal catalyst system promoted with noble metal. Unpromoted base metal catalysts have been described in U.S. Pat. No. 3,322,491 by Barrett et al., issued May 30, 1967.
Normally the activity of a noble metal carbon monoxide and/or hydrocarbon oxidation catalyst can be increased by maximizing the dispersion of the noble metals. However, when the catalyst is used in an auto exhaust stream where gas velocities are high and contact times are short, the availability of the noble metal appears to be more crucial to high acitivity than the extent of dispersion. The reasons for this is that the rate of oxidation of carbon monoxide and hydrocarbons may be diffusion controlled. It is well known that a pelleted or balled catalyst operating under very high space velocity conditions, makes use of only the outer 5 to 10% of its volume for catalyzing the oxidation of hydrocarbons and carbon monoxide. Therefore, the best possible catalyst as far as performance is concerned is one in which the noble metals are located in the outer 5 to 10% of the pellet volume and one that is highly dispersed.